Asymmetric doublets

For relaxation times t 1 ns, the spectra can be described as three Lorentzian lines with different line width, and for relaxation times around 10 ° s, the spectra appear as asymmetric doublets with line widths that decrease with decreasing relaxation time. In the theoretical spectra in Fig. 6.2, the EFG was assumed uniaxial... 

Figure 3 Creation of the longitudinal order by cross-correlation as a function of the mixing time fm which follows the inversion of a carbon-13 doublet (due to a./-coupling with a bonded proton). The read-pulse transforms the longitudinal polarization into an in-phase doublet and the longitudinal order into an antiphase doublet. The superposition of these two doublets leads to the observation of an asymmetric doublet.

An asymmetric doublet having a separation of about 130 G and centred near the free-spin value is often one of the components of the electron spin resonance spectra of irradiated organic solids. Because of this unusually large splitting, one can in general readily distinguish this doublet from other features of the spectrum (Pig. 3). Also, the spectrum is sufficiently simple for the magnitudes of the g and hyper-fine tensors to be accurately measurable (Adrian et al., 1962 Brivati et al., 1962). 

Deuterated PB networks filled with carbon black have been investigated recently [74]. The 2H NMR lineshape is different from that in unfilled elastomers an asymmetric doublet is observed as the sample is uniaxially stretched (A,=1.8)). This asymmetry is related to the presence of carbon black fillers, which induce magnetic inhomogeneities. 

Solid-state magnetic susceptibility data for 36 is consistent with little or no exchange coupling between the two ferrous centers (58, 59). The Mossbauer spectrum (zero field, 4.2 K) of 36 shows a broad asymmetric doublet consistent with two distinct sites (8i = 1.26 mm s 1, AEq = 2.56 mm s-1 82 = 1.25 mm s-1, A Eq = 3.30 mm s-1). EPR spectrum (X-band, 7 K) of the diferrous system contains a broad feature at g 16, indicative of an integer S = 4 spin state. 

Relative to polycrystalline adamantane (the least shielded resonance, 38.56 ppm) [37]and NH4CI [38,39] asymmetric doublets (Hz) ... 

Next, we describe the use of the poly(L-alanine) adopting the /3-sheet form to determine the NH proton chemical shift characteristic to /3-sheet conformation and to check the effect of conformation on NH proton signal broadening and line shape. As shown in Fig. 40(C) and (D), the half-width of the NH proton signals of H-[Ala]5-NHBu and [Ala ]n-1 was 2.4 and 1.5 ppm, respectively, which are similar to those of a-helical poly(L-alanines). The NH line shape of H-[Ala]5-NHBu exhibits a typical asymmetric doublet pattern, but which is quite different from that of the a-helical poly(L-alanine). The true NH proton chemical shift could not be determined from the normal singlet pattern of [Ala ]n-1 because of the low S/N ratio. 

From the H CRAMPS NMR spectra, therefore, it was possible to determine the NH proton chemical shift value for [Ala ]n-2 (a-helix 6 = 8.0) which is identical with that determined using BR-24 (2.0 kHz). Further, it was possible to determine the " NH proton chemical shift for [Ala ]n-1 (/3-sheet, 6 = 8.6) using the MREV-8 pulse sequence at 3.5 kHz. However, unfortunately, the NH proton chemical shift values for [Ala]n-2 and [Ala]n-t could not be determined because the line shapes of the " NH signals exhibit an asymmetric doublet pattern in this system also. Thus, it is found that determination of the true NH chemical shift of poly(L-alanines) can be achieved to measure fully N-labelled samples at higher MAS speed (3.5 kHz) and that these chemical shifts depend on conformation (a-helix 6 = 8.0 /3-sheet S = 8.6). This is the first determination of the true NH proton chemical shifts of poly(L-alanines) by H CRAMPS NMR. 

In the simple CP/MAS experiment for 15N and 13C nuclei in a peptide, scalar coupling to H is not observed because high-power ]H decoupling is used to remove the H-15N or H-13C dipolar interactions. However, for the carbons directly bonded to nitrogens, 14N-13C or, 5N-,3C dipolar interactions are not averaged out by MAS and broadened asymmetric doublets are observed.14-17 Accord-... 

This commonality in the radiolysis was established not only for cooked meats irradiated to high doses at -40°C, but also for raw meats irradiated to lower doses at 77 K [29]. The ESR spectra for raw pork, beef sirloin, and chicken breast (Figure 14) show the singlets associated with radicals formed by electron addition to the carbonyl groups, the yields of which linearly increased with dose. After annealing at -78°C, the spectral features changed to the predominant asymmetric doublet associated with the peptide backbone radical. Moreover, a direct comparison of spectra at -78°C for raw and roasted turkey breasts irradiated to 3.8 kGy showed no differences, indicating that native and denatured conformers of the protein respond radiolytically in similar ways [29]. 

Two mechanisms have been proposed to explain the appearance of an asymmetric doublet in randomly oriented substances with no magnetic ordering. One mechanism is based on the combination of the directional quantities — the angular distribution function of the magnetic dipole radiation and the Debye-Waller factor which becomes anisotropic in systems of lower than cubic symmetry. This mechanism predicts an asymmetry which should decrease as the temperature is lowered, in contradiction to the experimental observations in hemoglobin. The second mechanism is based on magnetic interactions described by the general Hamiltonian Eq. (234). 

The resonances from carbons directly bonded to nitrogen are asymmetric doublets in solid state NMR spectra (2i), This unusual llneshape results from the failure of magic angle sample spinning to completely suppress the C- N dipolar couplings, the success of which depends on the spins of... 

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